I wouldn't say any options are essential, what options to go for will depend on your budget and what you will find useful. The GTi is very well equipped as standard.
I have on mine reversing camera and the climate screen, at the time these were fairly inexpensive items that I would get some benefit from. All the others were either superfluous, or nice to have but really worth paying for.
As for how much to pay, a brand new manual Mk8 in white with 19's can be had for £29k and a DSG just over £30k. So for a 1yr old late Mk7.5 I would hope to be paying at least £3k to £4k less than that.